European Patent 1 524 186 discloses a device for collecting fluids having a density lower than that of the surrounding water and escaping from an underwater source. The device comprises an inverted-funnel-shaped dome placed over the underwater source for collecting the fluids escaped from the source and rising therefrom due to buoyancy. A transfer tube has a lower end in communication with the dome for transferring the collected fluids towards the surface. The device further comprises a submerged buffer reservoir, which is maintained at a predetermined depth under the surface, the submerged buffer reservoir being in communication with an upper end of the transfer tube for receiving the collected fluids. In the chamber of the submerged buffer reservoir, water will separate from the collected fluid due to the different densities, so that the submerged buffer reservoir acts as a separator, which concentrates the fluids in the upper part of the chamber.
U.S. Pat. No. 4,395,157 discloses an offshore drilling and pumping platform. The document shows collecting means in the form of an inverted funnel placed over an underwater oil and gas source, a riser tube for transferring the collected fluids towards a buffer reservoir maintained at the surface. The buffer reservoir comprises a chamber with an open bottom, into which the riser tube opens with its upper end. Gas is separated from oil in the buffer reservoir. GB 2066095 discloses a device for the recovery of fluids from underwater leaks. The device comprises a collection bell disposed above an underwater well, an open-bottom container arranged above the collection bell for storing the fluids and a riser, which reaches from the top of the collection bell into the interior of the container. At the upper end of the riser, a spiral flow channel is arranged for promoting the separation of the different phases of the fluids.
Document GB 2071020 relates to an apparatus for capturing and storing oil and gas flowing uncontrollably from an offshore well. The apparatus comprises an underwater liquid-gas separator to be positioned above a sea floor well blowout. Gas and oil is stored within a floatable storage vessel with an open bottom. Gas and oil are transported separately from the separator to the storage vessel, via an oil hose and a gas hose, respectively.
Document WO 81/01310 criticizes collecting means in the shape of sombreros (or inverted funnels), stating that it is difficult to transfer the mixture of oil and gas to the surface because of the proportional expansion of the escaping gas and pressure fluctuations due to gas bubbles. To address that problem, WO 81/0310 proposes to arrange a separator column on the seabed and to evacuate oil and gas though separate riser tubes.
The device disclosed in EP 1 524 186 has the advantage that it may be relatively easily installed above an underwater source. However, modifications to the system are necessary to enable it to deal with underwater sources releasing substantial amounts of gas, e.g. a borehole gone out of control, like the Mocondo well in the Gulf of Mexico in 2010. Indeed, as indicated by WO 81/01310, the expansion of the gas could destabilise the flow of the collected fluids up the riser tube. Nevertheless, a separation of the gas content from the other fluids at the seabed is undesirable, since it makes the system on the seabed more complicated. Valuable time may be lost in case of a blowout if a separator column as disclosed in WO 81/0310 must be put in place.